Pakistan is under growing pressure to free senior Taliban figures, particularly Baradar, to boost reconciliation efforts, as most Nato combat troops prepare to pull out of Afghanistan by the end of next year and anxiety grows over the country's security.

"In principle, we have agreed to release him. The timing is being discussed. It should bevery soon ... I think within this month," Sartaj Aziz, Pakistani Prime Minister Nawaz Sharif's adviser on foreign affairs, said.

Baradar's fate is at the heart of Afghanistan's efforts to kick-start the stalled peace process and push Pakistan to hand over senior Taliban captives who may provide leverage in the talks. But Aziz said Baradar would not be handed over to Afghanistan directly as some in Kabul had hoped and would instead be released straight into Pakistan.

The Afghan government believes Baradar is more open to dialogue than many of his comrades, but it is not clear whether he would promote peace or war against President Hamid Karzai's Western-backed government after his release.

One of the most ruthless Taliban figures, he was given his nom de guerre of "Baradar", or "brother", by Taliban leader Mullah Mohammed Omar.

Aziz said it was important to make sure the released Taliban prisoners had a chance to establish contact with their leadership on the ground to persuade them to be part of peace talks - an idea he said Karzai has agreed to.

"Obviously Karzai wanted him to go to Afghanistan, but we feel that if they are to play a positive role in the reconciliation process then they must do it according to what their own Shura [Council], their own leadership, wants them to do," he said.

"That they can't do unless they are released. ... I think he [Karzai] accepted this point that they should play a constructive role in the peace process."

Aziz's remarks followed last month's trip by Karzai to Pakistan, where he sought the handover of Afghan insurgents as part of the stalled peace process.

On Saturday, Pakistan freed a group of Taliban in an attempt to improve its troubled relations with Afghanistan, but once again risked angering Kabul by not handing them over directly.

Afghanistan fears Pakistan is only pretending to support dialogue while its intelligence agencies harbour Taliban leaders to project influence across their shared frontier.